8-1 COBOL for the 21 st Century Nancy Stern Hofstra University Robert A. Stern Nassau Community...

8-1

COBOL for the 21st Century

Nancy Stern Hofstra University

Robert A. Stern Nassau Community College

James P. Ley University of Wisconsin-Stout (Emeritus)

John Wiley & Sons, Inc.11th edition

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Decision Making Using the IF and EVALUATE Statements

Chapter 8

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Chapter Objectives

To familiarize you with

1. IF statements for selection

2. Formats and options available with conditional statements

3. EVALUATE statement

8-4

Chapter Contents

• Selection Using Simple IF Statement

• Selection Using Other Options of IF

• Using IF Statements to Determine Leap Years

• Condition-Names

• EVALUATE Statement: Using Case Structure as Alternative to Selection

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COBOL Statements

Two categories• Conditional statements

– Performs operations depending on existence of some condition

– Coded with IF-THEN-ELSE structure

• Imperative statements– Performs operation regardless of existing

conditions– MOVE, ADD are examples in COBOL

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IF Statement

IF condition-1

[THEN]

imperative statement-1 …

[ELSE

imperative statement-2 …]

[END-IF]

Format

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IF Statement

• If condition exists or is true– Statement(s) after THEN executed– ELSE clause ignored

• If condition does not exist or is false– Statement(s) after ELSE executed– Statement(s) after THEN ignored

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IF Statement Example

If Disc-Code = 1 Then

Multiply Amt By .15 Giving WS-Discount

Else

Move 0 To WS-Discount

End-If

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IF Statement Example

• If Disc-Code is 1, condition is true– MULTIPLY statement executed

• If Disc-Code is not 1, condition false– MOVE statement executed

• After selected statement executed, program continues with statement after END-IF

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ELSE is Optional

• May be omitted if operation required only when condition exists

If Acct-Balance < 0 Then

Display 'Account overdrawn'

End-If

• DISPLAY executed if Acct-Balance less than zero, otherwise it is ignored

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Relational Operators

Symbols for simple relational conditions

Symbol Meaning

< is less than

> is greater than

= is equal to

<= less than or equal to

>= greater than or equal to

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Condition Examples

Assume L, M and N are numeric

L = 12, M = 7, N = 3

Condition Result

L >= M True

M < 7 False

M > N + 6 False

M + N <= 10 True

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How Comparisons Performed

• Compare fields to others fields or literals of same data type

• Numeric fields compared algebraically005 < 026 < 539

• All of these considered equal012 12.00 12 +12

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How Comparisons Performed

• Nonnumeric fields compared alphabetically

ABLE < BAKE < BARK

• Blanks on right do not affect comparison

• All of these considered equal ABC ABCbb ABCbbbbb

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Collating Sequences

• When alphanumeric field has mix of upper-, lower-case letters and digits– Result of comparison depends on collating

sequence used on computer

• Two types of internal codes to represent data– EBCDIC mainly on IBM mainframes– ASCII on PCs, minis, non-IBM mainframes

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Collating Sequences

EBCDIC ASCII

Low Spaces Spaces

| Special characters Special characters

| a-z 0-9 A-Z A-Z

High 0-9 a-z

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EBCDIC vs ASCII Comparison

EBCDIC ASCII

g < G g > G

Cat < CAT Cat > CAT

D3 < 3D D3 > 3D

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CONTINUE clause

• Used to indicate no operation should be performed when a condition exists

If Amt1 = Amt2

Then

Continue

Else

Add 1 to Total

End-If

No operation performed if Amt1 = Amt2, continues with statement after End-If

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Nested Conditional

• IF statement itself can contain additional IF statements

• Pair each IF with an END-IF

• Used when more than two conditions need to be tested

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Decision Table

• Often used to list conditions and actions to be performed

Condition 1 Condition 2 Action

Code = 'T' N > 10 Multiply.15 By N

Code = 'T' N <= 10 Multiply.25 By N

Code not ='T'

N = anything N = 0

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Code for Decision Table

If Code = 'T'If N > 10

Multiply .15 By NElse

Multiply .25 By NEnd-If

ElseMove 0 To N

End-If

Delimits inner IF

Delimits outer IF

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Compound Conditional

• To test for several conditions with one statement

• Code multiple conditions separated by ORs or ANDs

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OR Compound Conditional

• Use OR to test whether any one of several conditions exists

If A = B Or B > 12Add A To Total

ElseAdd 1 To Count

End-If

Executed if either condition exists

Executed only if A not = B and B <= 12

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Implied Operands

• When same operand used in compound conditions, operand can be named once

• If X = 10 Or X = 20 may be written

If X = 10 Or 20– Tests two simple conditions, X = 10, X = 20– X is the implied operand in the second

condition test

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AND Compound Conditional

• Use AND to test if all of several conditions are met

If A = 5 And B > 0Add 10 To A

ElseMove 0 To B

End-If

Executed if both simple conditions met

Executed if one or both simple conditions not met

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AND and OR in Conditionals

• Compound conditions may include both AND and OR

• Hierarchy rules– Conditions with AND evaluated first from

left to right– Conditions with OR evaluated last from left

to right– Parentheses used to override this order

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AND and OR in Conditionals

If Q > 0 Or R < S And R = 10Multiply 2 By Q

End-If

Test conditions in this order:1. R < S And R = 10

OR 2. Q > 0

Example

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AND and OR in Conditionals

If Q = 2, R = 5, S = 16,

• Test in (1) R < S And R = 10 is false – Since R is not equal to 10 both conditions

not met

• Test in (2) Q > 0 is true – One or conditions surrounding OR is met– MULTIPLY statement will be executed

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AND and OR in Conditionals

• If conditions tested in this order result will be different

1. Q > 0 Or R < SAND 2. R = 10

If Q = 2, R = 5, S = 16

• Test in (1) is true

• Test in (2) is false - both conditions not met so MULTIPLY not executed

8-30

Sign Tests

• To test whether field is POSITIVE, NEGATIVE or ZERO

Condition ResultIf Amt Is Positive True if Amt is greater

than 0If Amt Is Negative True if Amt is less

than 0If Amt Is Zero True if Amt equals 0

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Class Test

• To test whether type of data if field is numeric or alphabetic

Condition Result

If Amt Is Numeric True if Amt = 153

False if Amt = 15B

If Code Is Alphabetic True if Code = PQR

False if Code = P23

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ALPHABETIC Class Tests

Reserved Word Meaning

ALPHABETIC A-Z, a-z, and blank

ALPHABETIC-UPPER A-Z and blank

ALPHABETIC-LOWER a-z and blank

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Negating Conditionals

• NOT placed before conditional reverses its truth value

Condition Result

If Amt Not = 10 True if Amt is 15

False if Amt is 10

If Amt Not > 8 True if Amt is 2

False if Amt is 12

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Negating Conditionals

These two conditions are not the same

• If Amt Is Negative– True if Amt is less than zero

• If Amt is Not Positive– True if Amt is less than or equal to zero– Zero (0) is neither positive or negative

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Negating Conditionals

These two conditions are not the same• If In-Code Is Numeric

– True if Code is digits only

• If In-Code Is Not Alphabetic– True if In-Code contains any character that

is not a letter– Field with combination of letters, digits and

special characters is neither NUMERIC nor ALPHABETIC

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Negating Compound Conditionals

• To negate compound conditional place it in parentheses, precede it with NOT

• Condition to check for In-Code of S or D

If In-Code = 'S' Or In-Code = 'D'

• To negate this condition (check for In-Code that is neither S nor D)

If Not (In-Code = 'S' Or In-Code = 'D')

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Negating Compound Conditionals

• May also use DeMorgan's Rule to negate compound conditions

• For conditions separated by OR change OR to AND and use NOT in each condition

• Condition to check for In-Code that is neither S nor D may be stated as

If Not In-Code = 'S' And Not In-Code = 'D'

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Negating Compound Conditionals

• To negate conditions separated by AND change AND to OR and use NOT in each condition

• Condition If A = B And C = D may be negated with either of these conditions

If Not (A = B And C = D)

If A Not = B Or C Not = D

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Condition-Names

• Meaningful names defined for specific values that an identifier can assume

Associate names with employee pay code values

Pay-Code Condition-name H Hourly S Salaried

Example

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Defining Condition-Names

05 Pay-Code Pic X.

88 Hourly Value 'H'.

88 Salaried Value 'S'.

• Define field in DATA DIVISION

• Use level 88 to define condition-name and associated value

Example

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Using Condition-Names

• Use any place a condition can be used in PROCEDURE DIVISION

If HourlyPerform Calc-Hourly-Pay

End-If

• If Pay-Code field has a value of 'H', condition Hourly is true

• Hourly same as condition Pay-Code='H'

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Using Condition-Names

• Condition-name must be unique

• Literal in VALUE clause must be same data type as field preceding it

• May be coded with elementary items with level numbers 01-49

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Using Condition-Names

• 88-level may specify multiple values

05 Opt-Num Pic 9.

88 Valid-Options Value 1 Thru 5

Valid-Options true if Opt-Num = 1, 2, 3, 4 or 5

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EVALUATE Statement

• Used to implement Case structure

• Tests for series of conditions

• May be used in place of IF statement

• Often code clearer, more efficient with EVALUATE when multiple condition need to be checked

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EVALUATE Statement

identifier-1EVALUATE

expression-1WHEN condition-1

imperative-statement-1 …[WHEN OTHER

imperative-statement-2 …][END-EVALUATE]

Format

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EVALUATE Example

• Add, subtract or multiply a number by 10 depending on value in Op-Code

Evaluate Op-CodeWhen 'A' Add 10 To NumWhen 'S' Subtract 10 From NumWhen 'M' Multiply 10 By NumWhen Other Display 'Code invalid'

End-Evaluate

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EVALUATE Statement

• When Op-Code is 'A' the ADD statement will be executed– Execution will continue with statement after

END-EVALUATE

• If Op-Code is not A, S or M, statement following When Other is executed

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Chapter Summary

• Simple relational conditons use the operators =, <, >, <=, >=

• Simple IF Statement– If condition exists, all statements up to

ELSE clause or END-IF are executed– If condition does not exist

• Statements after ELSE are executed• Next statement after END-IF executed if no

ELSE

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Chapter Summary

• Comparisons made – Algebraically for numeric fields– Using collating sequence for alphanumeric

fields

• Compound conditions join simple conditions with AND or OR– ANDs evaluated before Ors in order left to

right– Parenthese used to override hierarchy rules

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Chapter Summary

• Other tests– Sign tests - POSITIVE, NEGATIVE, ZERO– Class tests - NUMERIC, ALPHABETIC– Negated conditionals - may precede any

test with NOT

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Chapter Summary

• Condition-names may be defined at 88 level– Associates name with value a field may

assume– Use name as condition in PROCEDURE

DIVISION

• EVALUATE often used as alternative to IF or series of nested IFs

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Copyright © 2003 John Wiley & Sons, Inc. All rights reserved. Reproduction or translation of this work beyond that permitted in Section 117 of the 1976 United States Copyright Act without the express written permission of the copyright owner is unlawful. Request for further information should be addressed to the Permissions Department, John Wiley & Sons, Inc. The purchaser may make back-up copies for his/her own use only and not for distribution or resale. The Publisher assumes no responsibility for errors, omissions, or damages, caused by the use of these programs or from the use of the information contained herein.

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Decision Making Using the IF and EVALUATE Statements

Chapter 8